The present invention is directed to the field of interferometry.
Interferometers are widely used in spectrometers and in angle-measuring devices. An interferometer splits a beam into two parts, directs them along different paths, and then recombines them. Because of the different path lengths, the radiation in the two beams is not in general in phase, so interference occurs. In an angle-measuring system the lengths of the different paths are differently affected by the movement to be measured, and the intensity resulting from the interfering recombined light beams varies between a minimum and a maximum within a single-wavelength change in the difference between the two path lengths. By observing the intensity variations in the recombined light beams, therefore, one can detect minute motions.
FIG. 1 depicts a particularly simple interferometer arrangement of the type described in U.S. Pat. No. 3,348,446 to Young. A refractive block 10 provides on one of its faces first and second beam-splitting surfaces 12 and 14. Surface 12 splits an incoming light beam I into two components, represented respectively in FIG. 1 by dotted and dashed lines. A reflective surface 16 on the other face of refractive block 10 receives the dashed-line beam and reflects it, while the other, dotted-line beam is reflected by a mirror 18. Mirror 18 is spaced apart from refractive block 10 by a cylindrical rod 20. Mirror 18 and refractive block 10 are spring loaded so that the cylindrical rod 20 is compressed between them and maintains a separation equal to its diameter. The rod diameter is kept within tolerances tight enough to ensure that the interferometer is in adjustment. When mirror 18 tilts through an angle to be measured, the cylindrical rod 20 rolls and/or slides between its two lines of contact. The Young patent indicated that its teachings could be practiced not only in the arrangement of FIG. 1 but also in an arrangement in which it was the refractive blocks rather than the mirror that tilted. In practice only the moving-mirror version was used, however, since the moving part in the other version would have had a higher moment of inertia, and no countervailing advantage for that version had been identified at that time.
As FIG. 1 illustrates, the dotted- and dashed-line beams are reflected by the mirror 18 and the refractive-block reflective surface 16, respectively, to another refractive-block reflective surface 22 and the mirror 18, respectively, which in turn reflect the beams to the second beam-splitting surface 14, which reunites the beams for interference.
Although the Young arrangement provided a simple spectrometer that was easy to set up and maintain, it did have a drawback. As is stated in lines 67-72 of column 3 of the Young specification, the pivoting of the mirror resulted in some relative shifting of the two components of the recombined beam. This relative movement causes an undesirable reduction in the amount of overlap of the two "recombined" beams. In addition to the relative motion, the Young arrangement resulted in swinging of the recombined beam as a whole. Together, those two effects were annoying and, for some applications, intolerable.
An object of the present invention is to employ the teachings of the Young patent mentioned above without suffering the exit-beam shifting and swinging experienced by prior users of the invention.